The manufacturing of semiconductor devices such as integrated circuit (IC) chips utilizes semiconductor wafers. The semiconductor wafers serve as a substrate in which various three-dimension IC structures are formed using a series of processing steps. Once the IC chips are formed, they then need to be packaged, i.e., encapsulated in a support structure to form the finalized IC device.
One approach to the packaging process involves the use of reconstituted wafers. The size of the reconstituted wafer can be similar to a 300 mm silicon wafer, or it may be any reasonable size and shape. The reconstituted wafer is made of a resin. The resin is poured into a mold. While the resin is solidifying, known good IC chips (dice) are embedded in the surface of the resin in select positions. The resin is then allowed to solidify, thereby securing the IC chips in their select positions.
After the reconstituted wafer is fabricated, a lithographic process is carried out to electrically connect select IC chips. The lithographic process involves forming line features that can have widths in the range from 1 micron to 10 microns using a lithographic tool that has an exposure field and a depth of focus over the exposure field in the range from 1 micron to 20 microns.
Ideally, each IC chip is placed in a select (x,y) coordinate location with its surface at a select height h in the z direction and residing in the corresponding z-plane. One or more IC chips may fall within the exposure field. In some cases, one or more IC chips are used to define a module, with one or more modules falling within a given exposure field. The lithographic tool moves relative to the reconstituted wafer in a step-and-repeat manner or a step-and-scan manner to process all of the modules supported by the reconstituted wafer.
Under ideal conditions, the lithographic process can be readily carried out without much concern about whether the surfaces of the IC chips within a given exposure field fall within the depth of focus of the lithography tool. Unfortunately, there is usually substantial error in the positions of the IC chips relative to their ideal positions. For example, as an IC chip is placed in the resin, it can move slightly, e.g., can tilt up or down and can rotate. The IC chip may also sink into the resin or not sink far enough into the resin so that the heights of the IC chips are not all the same and not in the same z plane. The curing of the resin results in some amount of shrinkage that can result in movement of the IC chips. As a result, the surface of the IC chips within a given exposure field do not all reside in a common plane.